In recent years, slim type loudspeakers are drawing attention in order to cope with requirements for miniaturization and higher sound pressure of mobile telephones.
Because of these requirements, much effort is being made to configure magnetic circuits into a rectangular shape in order to improve the space factor. Also, these magnetic circuits generally employ an internal magnet structure. Furthermore, many loudspeakers use magnets such as rare earth magnets having a large magnetic energy in order to cope with requirement for higher sound pressures.
Now, a description of a conventional internal magnet type loudspeaker will be given referring to FIG. 13.
FIG. 13 is a cross-sectional view of a conventional internal magnet type loudspeaker.
Magnetic circuit 4 is formed by sandwiching rectangular magnet 1 with yoke 3 and upper plate 2. Magnetic circuit 4 is press-fit into and joined with frame 6 while maintaining yoke 3 of magnetic circuit 4 in contact with frame 6.
When performing this process, an adhesive is coated on the surfaces of contact.
Subsequently, diaphragm 7 is bonded to the rim of frame 6 followed by coupling voice coil 8 to diaphragm 7 for driving it.
Voice coil 8 is held inside magnetic gap 5 in this way. Japanese Laid-Open Patent Application No. Sho 59-193699 discloses a planar driving type loudspeaker having a high magnetic conversion efficiency. This loudspeaker employs a magnet having a large magnetic energy and the magnetic circuit employs an internal magnet structure.
However, recent market requires increasingly higher sound pressures. The conventional loudspeakers as mentioned above already use magnets having a high magnetic energy and also employ an internal magnet type magnetic circuit structure. For this reason, improvement in the magnetic energy by improvement of magnet materials has already reached the limit. As a result, the only means left of attaining a higher sound pressure is improvement of the magnetic energy by increasing the volume of the magnet. In an internal magnet type magnetic circuit structure, in order to increase the volume of the magnet, it is necessary to increase the size of the voice coil or the height of the loudspeaker. This is against the market requirement for miniaturization. Accordingly, the conventional loudspeakers suffer problems of not being able to satisfy incompatible requirements of higher sound pressure and miniaturization.
The present invention addresses these problems and provides a loudspeaker that can cope with higher sound pressure requirement while maintaining a small size.